1. Field of the Invention
The present invention relates to a centrifugal blower, and more particularly to a scroll casing of a centrifugal blower.
2. Description of the Prior Art
As illustrated in FIG. 1, a centrifugal blower according to the prior art includes a siroco fan 2 having a plurality of fins in peripheral directions, and a scroll casing spirally forming a draft passage at an external side of the siroco fan 2, where air is infused along an axle direction of the siroco fan 2 to form an eddy current in the scroll casing at a 90 degree direction relative to an air intake, to thereby be discharged via a discharge outlet 6.
A spiral of the scroll casing in the conventional centrifugal blower consists of an Archimedean spiral and a logarithmic spiral, where a spiral start point (P1) is situated at an inner side of a spiral chamber (S) from a line connecting a center (O) of the siroco fan 2 and a spiral end point (F1), such that a scroll angle is over 360 degrees.
Furthermore, as illustrated in FIG. 2, the Archimedean spiral is a curved line where a scroll radius (a distance from a center of an siroco fan to a spiral) is linearly increased as a scroll angle (an angle measured from a spiral start point to a spiral end point) is increased, and the logarithmic spiral is a curved line where a scroll radius is linearly and almost straightly increased as a scroll angle is increased.
There is a problem however in that a scroll casing manufactured by utilizing the Archimedean spiral or the logarithmic spiral can easily form a curved line and is easy in manufacturing but eddy current is dispersed at a certain range of a peripheral direction to thereby reduce a blowing efficiency such that performance of a blower is decreased to thereby increase a noise.
In order to complement those problems at the scroll casing manufactured by using the Archimedean spiral or the logarithmic spiral, Japanese laid-open patent No. Hei 6-117397 is disclosed where a spiral is correctively manufactured to have a non-linear shape.
As illustrated in FIG. 3, a spiral of a scroll casing 14 in the Japanese laid-open patent No. Hei 6-117397 constitutes a scroll radius formed in a certain expanded angle from a spiral start point (P2) to an optional scroll angle, (.theta..sub.1) where a scroll radius from the scroll angle (.theta..sub.1) to a spiral end point (F2) is greater than a scroll radius formed in a predetermined expanded angle from the spiral start point (P2) to the spiral end point (F2), and a scroll radius of the spiral end point (F2) is the same as a scroll radius formed in a predetermined expanded angle from the spiral start point (P2) to the spiral end point (F2).
In other words, as illustrated in FIG. 4, when a scroll angle of the spiral start point (P2) is given as O and a scroll angle of the spiral end point (F2) is given as .theta..sub.2, a scroll radius of spiral at a scroll casing 14 can be described in the following formula. EQU r(.theta.)=r.sub.0 +r.sub.1 exp(n.theta.)+R(.theta.),
where EQU R(.theta.)=0 in case of 0&lt;.theta.&lt;.theta..sub.1,
and EQU R(.theta.)=K.sub.1 sin{.pi./2.times.(.theta.-.theta..sub.1)/(.theta.-.theta..sub.2)} in case of .theta..sub.1 &lt;.theta.&lt;.theta..sub.2,
and where
r.sub.0 =distance between a peripheral surface of an siroco fan and spiral start point (P2), r.sub.1 =siroco fan radius, n=expanded angle and .theta.=a voluntary angle measured on the spiral start point (P2). PA1 a siroco fan having a plurality of blowing fans; and PA1 a scroll casing forming a spiral draft passage at an external side of the siroco fan, where, the siroco fan is so mounted as to allow centre of the siroco fan to be detached at a predetermined distance from centre of a spiral comprising the draft passage of the scroll casing.
A solid line in FIG. 4 represents a scroll radius formed in a predetermined expanded angle from the spiral start point (P2) to the spiral end point (F2), and a dotted line defines a scroll radius from the voluntary scroll angle (.theta..sub.1) to the scroll angle (.theta..sub.2) at the spiral end point. Furthermore, the spiral start point (P2) moves towards external side of the spiral chamber (S) on a line connecting the center (0) of the siroco fan 12 to the spiral end point (F2). The range of the scroll angle is below 360 degrees.
However, there is a problem in the scroll casing according to the Japanese laid open patent No. Hei 6-117397 in that it is very difficult to manufacture same in accurate non-linear curve according to the numerical formula. There is another problem in that frictional resistance between airs and between inner surfaces of scroll casing is not much reduced to decrease a blowing efficiency and performances of a blower and to thereby increase a noise.